1. Field
This disclosure relates generally to diode pumped laser systems and, more particularly, to diode pumped laser systems using multiple disks.
2. Description of Related Art
The principle of laser operation is stimulated emission of energy. When an electron is in an upper (i.e., excited) energy level of the laser material and a lightwave of precisely the wavelength corresponding to the energy level difference between the unexcited and excited states strikes the electron, the light stimulates the electron to move down to the lower level and emit a photon. This photon is emitted in precisely the same direction and phase with that of the incident photon. Thus, a light wave is established in the laser material, and if it can be made to travel back and forth through the laser material (also known as the gain media) it will retain its frequency and grow in amplitude as it stimulates photon emissions.
This positive feedback mechanism is typically accomplished by a mirror placed at each end of the lasing material to reflect the traveling wave back through the lasing material. The rear mirror is fully reflecting, and the front mirror is partially reflecting and partially transmitting at the laser wavelength. Light reflected back and forth from the front and rear mirrors serves as positive feedback to sustain oscillation, and the light transmitted through the front mirror is the laser output light. The two mirrors are parallel and form an optical cavity that can be tuned by varying the spacing between them. In the case where the laser cavity is configured as a standing-wave resonator, the laser operates only at those wavelengths for which a standing-wave pattern can be set up in the cavity, i.e., for which the length of the cavity is an integral number of half wavelengths.
Laser systems generally comprise laser materials having four energy levels (a four-level system) or three energy levels (a three-level system). A material which has four energy levels involved in the lasing process is significantly more efficient than a material having three energy levels. At room temperature for a four-level system, the electron ground state is almost entirely occupied, while the lower laser level and the upper energy levels are essentially unoccupied. When the upper energy level has a greater electron population than the lower level, a population inversion exists. This inverted population can support lasing since a traveling wave of the proper frequency stimulates downward transitions of the electrons with the associated energy release.
The process of exciting the laser material to raise the electrons to an excited state, i.e., producing a population inversion, is referred to as pumping. Pumping can be accomplished optically with a flash lamp driven at a high frequency, by an electric discharge, by a chemical reaction, or, in the case of a semiconductor laser, by injecting electrons into the upper energy level with an electric current. When a sufficient number of electrons are in an excited state, the laser energy can be released by allowing the traveling wave to exit the laser cavity.
Many types of lasers exist in the prior art, including slab and disk laser systems, which use various laser materials. For many applications, the power scaling capability-and/or gain of such systems is inadequate. It would thus be useful to provide a laser system with improved power scaling and/or gain.
One of the laser materials used in slab and disk laser systems is Nd:YVO4. This material, with its broad pump bands and high gain, may be used advantageously in systems which are required to perform over a wide range of ambient temperatures, as well as in systems requiring short Q-switched pulse durations at high repetition rate, or high gain. However, because of the thermomechanical fragility of Nd:YVO4, laser systems using this material are very difficult to scale in power. Prior art systems using Nd:YVO4 typically consist of slugs (short rods) with diffusion bonded end caps, edge-pumped slabs, or rods. In such prior art systems, even with compensation for thermally induced aberrations, power scaling beyond a few tens of Watts is usually not possible.
One type of laser uses face-pumped disks, which provide an attractive pumping and extraction geometry because they minimize thermally induced optical aberrations in the beam propagation direction. However, the disks suffer from parasitic lasing and power scaling limitations. Power scaling may be accomplished by using multiple disks. However, accessing each one of the disks at near-normal incidence with the extraction lasing and the pump beams is difficult to achieve. Indeed, in order to limit thermally induced optical aberrations, prior art systems require that the extracting optical beam impinge on the disks at near normal incidence. This is because thermally induced aberrations are minimized when the optical path is parallel to the heat-flow, which is typically perpendicular to the disks.
Spectra-Physics of Mountain View, California has designed a laser apparatus using multiple end-pumped laser rods known as the Inazuma Periodic Resonator. This design is depicted in FIG. 1. In FIG. 1, the laser apparatus 10 comprises at least two laser rods 16 made of Nd:YVO4 through which a resonant laser beam 21 is directed in a zig-zag fashion. An aperture 14 assists in coupling the resonant laser beam 21 from one laser rod 16 to the other laser rod 16. The resonant laser beam 21 resonates between a high reflector 11 and an output coupler 12 and is directed into the ends of the laser rods by dichroic mirrors 13. Fiber coupled pump light 25 is coupled to the laser rods 16 by imaging optics 15 directing the pump light 25 through the dichroic mirrors 13 and into the ends of the laser rods 16. The output 23 of the laser apparatus 10 is produced by the output coupler 12. Additional laser rods 16 may be used to allow the output of the laser apparatus 10 to be scaled to higher powers.
As can be seen from FIG. 1, the Inazuma Periodic Resonator requires a rather complicated geometry for providing pump energy to the laser rods. In particular, the dichroic mirrors 13 must accurately direct both the resonant laser beam 21 and the pump light 25 into the laser rods 16. Further, the laser rods 16 must be cooled or coupled to a heat sink to direct heat out of the laser rods 16.
Another laser apparatus using multiple laser disks is described by H. Hügel and W. L. Bohn in “Solid State Thin Disk Laser,” Proc. SPIE-Int. Soc. Opt. Eng. (USA), Vol. 3574, 1998, pp. 15–28. The Hügel reference describes the use of multiple discs in order to allow scaling of the laser to higher power. FIG. 9 depicts an example of a multiple disk laser apparatus 900 as described in Hügel. FIG. 9 shows two laser disks 910 mounted on heat sinks 920. The apparatus 900 further comprises a reflector 930 and an output coupler 940. These elements are disposed to allow a laser beam 950 to propagate in a zig-zag manner within the apparatus 900 and then to be output at the output coupler 940.
Hügel does not specifically describe a preferred laser pump mechanism for use with the apparatus depicted in FIG. 9, but Hügel generally describes quasi-longitudinal or radial pumping schemes. The Hügel reference primarily addresses the thermal effects of pumping active laser medium encountered in scaling the laser power to higher powers. However, in the approaches described by Hügel, several problems remain unresolved, including parasitic oscillation.
Therefore, there exists a need in the art for a laser apparatus that provides for improved power scaling and/or gain without complicated optical devices or a high number of components. Further, the laser apparatus should provide for power scaling while minimizing parasitic oscillations and losses. Finally, the laser apparatus should provide compensation for thermally induced aberrations.